Imaging M31 Advice

[bendiddley]

Hi all. I'm about to embark on my first deep sky imaging session using a recently acquired EQ3-2 with dual motors. I'm going to be using a DSLR only at 300mm. Can you give me your advise regards ISO, exposure duration and number of subs/darks I would need to take and anything else to bear in mind? Cheers.

[Owmuchonomy]

I used F4 for 120 seconds at ISO 800 for my lights. I took 19 lights, 20 darks, 20 bias and 20 flats.  Final image is on my Flickr link below.  More subs than this equals better S/N.

[bendiddley]

I used F4 for 120 seconds at ISO 800 for my lights. I took 19 lights, 20 darks, 20 bias and 20 flats.  Final image is on my Flickr link below.  More subs than this equals better S/N.

Cool thanks. Like the image, hope I can get one like that. So you reckon if I took more than 19 it would be improved, how many more do you reckon? Also will not doing flats make a huge difference, I haven't experimented with flats before and was thinking of not bothering at least to start anyway.

[bendiddley]

Cool thanks. Like the image, hope I can get one like that. So you reckon if I took more than 19 it would be improved, how many more do you reckon? Also will not doing flats make a huge difference, I haven't experimented with flats before and was thinking of not bothering at least to start anyway.

I've just realised that my lens only goes down to F5.6 at 300mm. What differences would I have to make to the settings you've suggested to compensate for this?

[Owmuchonomy]

I think you will be limited by your mount capability for sub length so you may have to go up to ISO 1600. Yes flats can make a difference so you should do them.

[jambouk]

I found i needed to stop down my lens else there was lots of artefact like coma at the edges. I'd read up that it should be stopped down two stops. This obviously has consequences of letting less light in and as such either the ISO needs to go up, or the exposure time increased, or the number of subs needs to be greatly increased.

James

[dph1nm]

This obviously has consequences of letting less light in and as such either the ISO needs to go up, or the exposure time increased

Increasing ISO does not compensate for less light - all it does it change the gain (i.e. multiplies both signal and noise by the same amount).

NIgelM

[jambouk]

Yes, i should have qualified increasing the ISO doesn't increase "sensitivity", but does in part "compensate" for less light but at the expense of making noise more evident and resultant images more grainy.

James

[Knight of Clear Skies]

I found i needed to stop down my lens else there was lots of artefact like coma at the edges.

This depends on the lens, a few are usable wide open while others give bad coma without stopping down. With the 300mm zoom it's probably going to be a case of compromising between star shapes and light gathering ability. I did have a quick try at Andromeda with my 250mm lens once, a single 1 minute sub wide open (f5.6). The coma is fairly obvious but M31 shows up quite well - if I'd stopped it down I'd have got far less signal on my target. Cropping down the image could be another option.

[bendiddley]

This depends on the lens, a few are usable wide open while others give bad coma without stopping down. With the 300mm zoom it's probably going to be a case of compromising between star shapes and light gathering ability. I did have a quick try at Andromeda with my 250mm lens once, a single 1 minute sub wide open (f5.6). The coma is fairly obvious bad but M31 shows up quite well - if I'd stopped it down I'd have got far less signal on my target. Cropping down the image could be another option.

If you were to stop it down on your 250mm, what would you stop it down to? Also what do you mean you got far less signal on your target? Sorry I'm new to all this. Cheers.

[Alien 13]

I have the 300mm Kit zoom and to be honest its ok wide open as is the 18-55mm one I did only try it at around 200mm though but I would start with It wide open and an ISO of 800-1600 and see what kind of results you get.

The exposure time needs to be kept around 90 seconds max unless you are confident that your mount is perfectly aligned/balanced etc.

Alan

[jambouk]

My tamron 70-300 zoom produces awful coma.

James

[Knight of Clear Skies]

If you were to stop it down on your 250mm, what would you stop it down to? Also what do you mean you got far less signal on your target? Sorry I'm new to all this. Cheers.

No problem, it's all a learning curve. Stopping down blocks some of the light reaching the sensor, requiring longer exposure times to compensate. F-number is an exponential scale, for example at f5.6 the lens transmits half as much light as at f4.0.

I'd have to experiment to get the best results out of my 250mm lens, I've only tried a couple test shots with it. It's not a fast lens and at 250mm I'd be limited to short exposures, so I'd probably try and use it wide open (f5.6) or at f6.3 in order to pick up the fainter region. I could then either crop the final image or try and correct any distortion in post processing. Your 300mm lens might be better or worse than my 250mm when it comes to aberrations.

[rockinrome]

Hi. Don't forget you will definitely need to Polar Align the mount before attempting any lengthy exposures.

I have the same mount head with the SynScan upgrade and have had no trouble with ISO 800 up to 120 secs ..... and I *think* it could go above that easily.

All the best.